One aspect of the invention relates to a system for providing an isolated supply of electrical power for an electrical load.
In general, an electrical load is supplied with electrical power in the form of a supply voltage or a supply current via a single electrical cable from a voltage source or current source.
Sometimes, an isolated (=DC isolated) power supply is required for the load, without any conductive connection between the load and the supply source. If the electrical load is arranged at an electrical potential which differs to a very major extent from that of the power source, or very stringent requirements with regard to electromagnetic compatibility (EMC) have to be satisfied, supply via a single electrical supply line is at least difficult. Examples of applications such as these are current measurement and voltage measurement in public power supply facilities, and ECG, breathing or pulse measurement on a patient during a magnetic resonance imaging investigation carried out in parallel.
A system is known in which light from a light source, for example from a laser diode or a light-emitting diode (LED), is transmitted to a photoelectric transducer, where it is converted into electrical energy, which is used to supply the load. In order to transmit light, the light source and the photoelectric transducer are optically connected to one another via an optical waveguide or else via a free-beam system. An isolated, optically operated supply system such as this for an electrical sensor is described, for example, in “Sensors and Actuators A” Volumes 25 to 27 (1991), pages 475 to 480. The light from a laser diode is transmitted via an optical waveguide to a photoelement array, which converts it to the electrical energy for the sensor. The measurement data from the sensor is likewise transmitted optically via an optical waveguide. However, owing to the special components that are used, in particular the high-power laser, the photoelement array and the optical plug connections, this supply system is associated with not inconsiderable complexity.
A system is also known, in which the electrical energy is obtained from the immediate area surrounding an electrical sensor that is to be supplied, for example inductively from a high-voltage network or photovoltaically from solar radiation. However, this system for supplying power is subject to the undesirable side effect that no electrical power is available when the high-voltage network is not operating, or when the sun is not shining. A system which draws its electrical power from the high-voltage network is described in DE 25 46 694 A1.
Another system for providing an isolated supply of electrical power for an electrical load is disclosed in DE 44 42 677 A1. In this case, a radio transmitter transmits energy in the form of radio waves to a radio receiver, which converts the radio waves to the electrical power for the electrical load. If the load is a sensor, then the measurement data from the sensor is also transmitted by radio. This system thus allows DC-isolated power and data transmission without any cables. Legal regulations relating to radio traffic restricts the capability to use this system, however.
A further known approach for an isolated power supply for a load which is, in particular, at a high-voltage potential is to supply radio-frequency electrical energy to a capacitor which is resistant to high voltages, or to a capacitive divider which may be present in any case for voltage measurement. The capacitor or the capacitive divider in this case bridges the potential difference. One disadvantage is that undefined impedance ratios may occur. This is because, while the capacitor or the capacitive divider is used as the forward line for the radio-frequency power, the return line is provided in a largely undefined manner via a conductor to ground capacitance of an existing high-voltage overhead line and/or via an adjacent item of equipment. In order nevertheless to ensure operability, a relatively high supply frequency, for example of >10 MHz, is required. However, the system then overall acts as a transmitting antenna, which on the one hand leads to an undesirable loss of energy due to radiated emissions, and on the other hand leads to conflicts with the already mentioned legal regulations relating to radio traffic.
A radio-frequency signal is also transmitted by capacitive components in the system disclosed in DE 910 925 for controlling the drive for gas and steam discharge paths. A first capacitive branch element is provided for the forward direction, and a second capacitive branch element for the backward direction. The radio-frequency signal is, however, not used for supplying power but, in fact, for controlling an initiation circuit which is associated with the high-voltage potential.
DE 29 11 476 A1 describes a further development of the transmission path disclosed in DE 910 925 for a system for supplying power to a load that is at a high-voltage potential. The transmission path is in this case in the form of a symmetrical filter chain formed by series capacitors and parallel inductors. The parallel inductors were expressly also included in the system in order to reduce the load on the radio-frequency generator from the provision of the wattless component, and in order to compensate for isolation differences occurring between adjacent capacitors. However, owing to the large number of individual components required, the system is complex to implement.